A rolled aluminum alloy sheet has been produced by: semi-continuously casting of a molten alloy product adjusted to a predetermined composition into a rolling ingot; slab cutting; homogenization step; surface cutting step; heating; and hot rolling. Cold rolling is performed when so required after the hot rolling. In such a rolled aluminum alloy sheet production process, a predetermined shape is achieved while melting coagulated structures, and adjustment for obtaining a homogenous and fine structure is performed. Also, in the rolling step, quality control (e.g. heat treatment) that is varied depending on the alloy type has been performed. Since there are the various rolled aluminum alloy sheet production steps as described above, there has been a limit in reduction in energy consumption and reduction in cost.
In recent years, a continuous casting method for aluminum alloy has been studied. This method is a method for continuously and directly casting a sheet material having a predetermined thickness from a molten metal of an aluminum alloy. In the continuous casting method, it is possible to continuously cast a sheet material having a thickness of 10 mm or less, for example, which is thinner than a slab. Therefore, a cooling rate is higher than that of the conventional ingot continuous casting, thereby obtaining a finer cast structure. Also, due to the high cooling rate, an allowable amount of Fe which has ordinarily been treated as an impurity element is increased, and recyclability of aluminum alloy is improved. Further, since it is possible to largely reduce the number of production steps, cost reduction can be achieved.
A 5000-system (Al—Mg) aluminum alloy has been mainly used as the rolled aluminum alloy sheet for an automobile outer panel, for example. As other examples, use of an excessive Si type 6016 alloy or 6022 alloy (Al—Mg—Si alloy) having a bake-hardening property has been studied in recent years. As used herein, “bake-hardening” means an aging phenomena utilizing heat in a baking step for an automobile.
For instance, in the Al—Mg—Si alloy, a material on which only a solution treatment has been performed (refinement: T4) is press-molded into a predetermined shape, and hardening is performed in a subsequent baking step, thereby obtaining the rolled aluminum alloy sheet for outer panels. Among aluminum alloys, the 6000-system alloy, such as the Al—Mg—Si alloy has strength and good corrosion resistance and has been used as an underbody material of an automobile or the like. In the 6000-system alloy, since such excellent properties are integrated with the above-described bake-hardening property and continuous casting and rolling, energy consumption for production is further reduced, thereby obtaining a highly functional and low cost material (see Patent Documents 1 to 3).
The level of strength of an aluminum alloy depends much on an alloy composition. Particularly, examples of an aluminum alloy capable of expressing high strength include a heat treated alloy which is precipitation strengthened by an aging treatment, and representative examples thereof include a 7000-system alloy (Al—Zn—Mg alloy) and a 2000-system alloy (Al—Cu alloy). The 6000-system alloy also belongs to this type but is inferior in strength properties as compared with other heat treated alloys. Meanwhile, a high strength 6000-system alloy to which Cu is added is under development.
However, like the 2000-system alloy and the 7000-system alloy, the Cu-added 6000-system alloy has a problem of reductions in processability and corrosion resistance despite the improvement in strength. Therefore, it has been difficult from the practical point of view to adapt the rolled aluminum alloy sheet made from such aluminum alloy to the outer plate, underbody, or the like of automobile to which corrosion resistance is required.
Also, the strength is improved by addition of an additive element in the rolled aluminum alloy sheet as described above, however, in an Al—Fe—Ni alloy or the like, for example, softening resistance is insufficient, and hardness (residual hardness) after casting, annealing, and long time heating, is subject to a large reduction as compared with the hardness after casting despite its heat resistance, i.e. its excellent strength at high temperatures. Therefore, it is impossible to perform high temperature aging on such aluminum alloy, and, consequently, since strength at room temperature is reduced after the high temperature heating despite the excellent strength under high temperature environments, it has been difficult to use such an aluminum alloy for parts to be used under high temperature environments.
As described above, as the rolled aluminum alloy sheets constituting automobile structural parts and the like, there has been a demand for the one that is capable of being molded into various desired shapes and excellent not only in strength, corrosion resistance, and the like, but also in softening resistance and the like.
It has been quite difficult to industrially produce a rolled aluminum alloy sheet that satisfies these property requirements using the aluminum alloy conventionally used.
Patent Document 1: JP 8-165538 Unexamined Patent Publication (Kokai)
Patent Document 2: JP 2004-156117 Unexamined Patent Publication (Kokai)
Patent Document 3: JP 2006-249550 Unexamined Patent Publication (Kokai)